As the prosecution tried to prove the guilt of Nicky Jacobs for the murder of PC Keith Blakelock, it tried to give the jury an idea of the terrifying events of 6 October 1985 on a rundown housing estate in north London.

Violence erupted on the Broadwater Farm estate in Tottenham after a local woman, Cynthia Jarrett, died while police searched her home following the arrest of her son. There was hand-to-hand combat between the police and rioters, and gunshots were heard. Youths on the estate would brag of giving police a "bloody good hiding".

Among a group of officers known as serial 502 was community officer Blakelock. He and other officers were asked to offer protection to firefighters trying to put out blazes that broke out as petrol bombs and missiles rained down.

Officers on duty that night testified about their ordeal. Sgt David Pengelly told the jury how he and colleagues were trapped by masked youths and ended up in a chaotic retreat, fighting for their lives.

"It was an absolutely terrifying scenario," he said. "We were in the middle of an estate, an unknown number of people were intent on doing us harm at least. As we retreated down the stairway there were people with iron bars and at least one with a machete effectively trying to chop the shields to bits. The retreat to ground level was just a continuing nightmare.

"All the time people [were] throwing themselves, hammering at our shields and it was my belief that if we didn't go faster down that stairway then that's where we would've been trapped."

On a grass verge Pengelly saw that a person was surrounded. He realised it was an officer who had become separated from the rest. A colleague shouted at the officer: "Move, we've got to move."

Mortally wounded, a knife embedded in him, Blakelock, 40, still could move his legs. He staggered a final few yards and collapsed.

The horror of what befell Blakelock was never in dispute, nor the unique ferocity of that riot. The challenge for the prosecution was proving that Jacobs, then 16, now 45, was behind some of the 43 wounds Blakelock suffered.

On Wednesday, after deliberating for nearly seven hours, a jury cleared Jacobs of stabbing Blakelock by a majority 10-2 verdict.

The crown had five key strands of evidence. Three people claimed they witnessed the murder. Two of them admitted attacking Blakelock but not stabbing him. They were given immunity from prosecution and anonymity as they testified.

The least problematic of the three witnesses was referred to as Q. He was not involved in the violence but claimed to have seen Jacobs stab Blakelock. He came forward in 2009.

Q, who admitted heroin use and heavy drinking, was watched by the jury as he testified. In an unusual move, jurors passed a note to the judge asking whether the witness had Korsakoff's syndrome, by which heavy drinkers suffer brain damage and subsequently invent things. According to the Alzheimer's Society, one symptom is that "a person invents events to fill the gaps in memory",.

Jacobs's barrister Courtenay Griffiths QC said he was not in a position to give an immediate answer to the question.

Q wrongly claimed that the murder took place in a concrete car park area under a housing block, which he thought was called Martlesham. It happened on a grassy area by Tangmere block.

The Tangmere block on the Broadwater Farm estate. Photograph: PA

John Brown, one of the two witnesses who admitted attacking Blakelock, was offered up by the prosecution as having identified Jacobs as an attacker armed with a machete or scythe.

But in a 1993 police interview he said black people looked the same to him. "It's very hard for me because, like, I'm not a racist person but to me a black is a black, all right? I can't tell the difference between them. To me a black man is a black man."

In the witness box this year he was pressed by Griffiths on whether this was still his view. Brown replied: "More or less."

The third witness, known as Rhodes Levin, named Winston Silcott as the ringleader of the attack in a 1985 police interview. Later he said he had made this up to tell police what he thought they wanted to hear.

Levin said that during the attack on Blakelock he was stood next to Jacobs, who was armed with a knife less than six inches long, contradicting Brown's claims that the defendant was armed with a machete.

The two other key elements of the crown's case were an alleged admission by Jacobs in 2000 when he was arrested for a separate matter. The jury heard that he was alleged to have told a police officer: "Fuck off, I was one of them who killed PC Blakelock."

Then there was a rap poem written by Jacobs in 1988 when he was in prison for affray during the riots, which the crown said the jury could interpret as an admission of involvement in the attack. It read in part: "We chopper we start chop him on his hand, we chop him on him finger, we chop him on him leg, we chop him on his shoulder him head him chest him neck, we chop him all over, when we done kill him off lord er feel much better."

Opening the case, the prosecutor Richard Whittam QC urged the jury to look at the totality of the evidence, and outlined some of the sorry history of the case.

Before Jacobs, six people had been charged and eventually cleared of the murder. The case against three juveniles in 1987 never got to a jury. In 1991 the appeal court quashed convictions against Silcott, Engin Raghip and Mark Braithwaite after new tests raised concerns about the reliability of the evidence and claims it was fabricated.

The officers in charge of the investigation were then put on trial and themselves cleared.

Police began a fresh investigation using new methods, and a decision was made that evidence against those who stabbed Blakelock was most likely to be obtained from those who were present.

Those who attacked Blakelock were categorised in two groups: those who had kicked or punched the officer, and those who used potentially lethal weapons to assault him. The "stabbers" were to be pursued, said Whittam, if needs be with the testimony of the "kickers".

"Were it not for the immunity, they [some of the witnesses] would have been prosecuted for murder," Whittam told the jury. He added: "There are aspects of all the principal prosecution eyewitnesses' characters of which you are likely … strongly to disapprove."

During their time as witnesses, he said, they had been "provided with some degree of financial assistance by police" for their "willingness to give evidence and co-operation". He said payments were "subject to a strict authorisation procedure and are limited".

Police on the Broadwater Farm estate the morning after the riot. Photograph: Julian Herbert/Getty Images

What was not said to the jury by the crown was that the police's actions after Blakelock's murder, including charging six people with murder, all of whom were eventually cleared, had made it harder for his killers to be brought to justice.

During the 1980s a series of riots struck inner-city Britain as economic crisis exacerbated tensions between police and the communities they served. There were disturbances in Birmingham, Bristol, Liverpool and Brixton. Broadwater Farm was the worst.

The former Met police deputy assistant commissioner Brian Paddick, now a peer, told the Guardian: "The police felt after 1981 that they had lost control of the streets. To have more riots where one of our own officers was killed was rubbing salt into wounds that had not healed after 1981."

Paddick served on the streets during the 1981 Brixton riots. He said: "Then, slabs and bricks and petrol bombs were thrown at the police from a distance. At Broadwater Farm, because of the terrain, the walkways, it was close-quarters battle."

After the butchering of Blakelock, Paddick recalled the questions some officers asked: "Do they hate us that much, that they would kill a police officer, a community officer?"

It was worse than that. Even those who may have known of the guilty distrusted the police so much that they wouldn't tell.

Despite the extreme violence, there was no official inquiry. Haringey council commissioned the Labour peer Lord Gifford to chair an investigation. Launching his report in 1986, he said: "The police came to regard this community as the enemy, but the police were wanted. The police should get together with the community associations and find out what they want, which isn't necessarily 'soft policing' – for example, if there were drug dealers in Broadwater Farm."

Gifford said that after the riot there was a "complete failure by police to recognise its causes", and noted the large numbers of officers camped on and around the estate afterwards. "On 10 October there were 9,165 officers in readiness, in November there were over 3,500 and in December over 1,500."

The events on Broadwater Farm in 1985 offer lessons and warnings relevant to today. A maxim of British policing is the idea that the police are the public and the public are the police, consenting to the rule of law. Before, during and after the riots, too many on both sides forgot this.

Those who took part in the murder and butchering of a police officer have never been punished for the crime. Three decades on, the case of PC Keith Blakelock is still not resolved.